This invention relates to a polarized beam splitter capable of efficiently splitting a polarized beam into S-components and P-components over an entire visible spectrum, and an illumination optical system and a projector provided with such a polarized beam splitter.
A polarized beam splitter splits a plane wave into two components: S-component and P-component by reflecting the S-component of the plane wave incident on a polarizing multilayered film at a specified incident angle and causing the P-component thereof to transmit. Known polarized beam splitters include a prism type one in which the polarizing multilayered film is held between a pair of prisms and a plate type one in which an incident angle of the plane wave on the polarizing multilayered film provided on a flat base plate becomes a Brewster angle.
There have been proposed a variety of apparatuses in which a polarized beam splitter is incorporated as an element in order to utilize the aforementioned polarized light splitting characteristic, i.e. the characteristic of splitting the plane wave into the S-component and P-component, thereby displaying an excellent performance in cooperation of the polarized beam splitter. One of such proposed apparatuses is, for example, a liquid crystal projector.
The liquid crystal projector is an equipment for projecting a large image on a screen. An image having a shape similar to that of an image desired to be projected is formed on a liquid crystal panel, and an illumination light aligned to a specified polarized light component by an illumination optical system is produced and projected on the liquid crystal panel. A light emitted form such a liquid crystal panel is introduced to the screen via a projection lens, thereby projecting an enlarged version of the image on the liquid crystal panel on the screen. In other words, in this liquid crystal projector, the illumination optical system is constructed by a light source, the polarized beam splitter and a half-wave plate, and the light from the light source is split into an S-component and a P-component by being incident on the polarized beam splitter at a specified incident angle. Thereafter, only one component, e.g. only the P-component is caused to transmit the half-wave plate, thereby converting it to S-component to produce a light aligned to the S-component. This light is projected to the liquid crystal panel as an illumination light.
In order to project a color image onto the screen, it is necessary to prepare liquid crystal panels for each of three primary color components (R(red)-component, G(green)-component and B(blue)-component) and to illuminate the respective liquid crystal panels with corresponding illumination lights. To this end, a white light source needs to be used as a light source and a light from the white light source needs to be split into S-component and P-component by the polarized beam splitter over the entire visible spectrum (400 nm to 700 nm). Further, this splitting efficiency, i.e. a splitting ratio is closely related to the intensity of the illumination light to the liquid crystal panel. When the splitting ratio decreases, the intensity of the illumination light actually projected on the liquid crystal panel decreases. This darkens the image projected on the screen, with the result that a satisfactory image cannot be obtained. Accordingly, the polarized beam splitter used in the color liquid crystal projector needs to have a polarized light splitting characteristic of efficiently splitting the incident light into S-component and P-component over the entire visible spectrum.
An incident angle .theta. of the light from the white light source on the polarizing multilayered film is normally set at 45.degree., and the illumination optical system is designed such that the splitting ratio is at maximum at this incident angle .theta.. However, in reality, not only the rays at incident angle .theta., but also rays displaced from those rays by about .+-.3.degree. are incident on the polarizing multilayered film. In the conventional polarized beam splitters, no sufficient investigation and consideration are made for such incident rays oblique to those incident at 45.degree.. If the oblique incident rays are incident on the polarizing multilayered film, the splitting ratio is considerably reduced, causing problems similar to the above.
Further, even if the liquid crystal panel surface is uniformly illuminated, no satisfactory image may not be obtainable due to a nonuniform luminance of an optical image projected on the screen.